The invention relates to an illumination system for supplying a polarized radiation beam, which system successively comprises a radiation source unit and an optical system for concentrating the radiation to a beam, a polarizing system for splitting a radiation beam generated by the radiation source unit into linearly polarized sub-beams, and at least one polarization-rotating element. The polarizing system includes at least one optically transparent plate having a refractive index n.sub.0 and a first face and a second face, while at least one of the faces is provided with an optically thin layer having a refractive index n.sub.1 which is larger than n.sub.0. The light path of at least one of the sub-beams incorporates a reflector for reflecting this sub-beam into the same direction of propagation as that of the other sub-beam. The chief ray of said beam and the normal on the plate enclose an angle which is substantially equal to the Brewster angle which holds for the interface between the optically thin layer and a medium surrounding the polarizing element.
The invention also relates to an image projection device comprising such an illumination system.
An illumination system of the type described in the opening paragraph is known from PCT patent application WO 96/05534. The illumination system described in this document comprises a polarizing beam splitter which, in combination with a polarization-rotating element, substantially completely converts the unpolarized radiation beam emitted by the radiation source into a radiation beam having a single linear direction of polarization. The beam splitter consists of an optically transparent plate, at least one face of which is provided with an optically thin layer having a refractive index n.sub.1 which is larger than that of the plate. The light path of at least one of the sub-beams incorporates a reflector for reflecting this sub-beam into the same direction of propagation as that of the other sub-beam. Furthermore, the chief ray of the beam of the radiation source and the normal on the plate enclose an angle which is substantially equal to the Brewster angle which holds for the interface between the optically thin layer and a medium surrounding the polarizing element.
The polarizing element is known per se, for example from the article: "Polarizing beam splitters for infrared and millimeter waves using single-layer-coated dielectric slab or unbacked films" by R. M. A. Azzam in Applied Optics, vol. 25, No. 23, December 1986.
A polarization-rotating element is present in one of the sub-beams. The polarization-rotating element ensures that the two components have the same direction of polarization so that substantially the complete beam of the radiation source is converted into a beam having the same linear direction of polarization.
A short-arc gas discharge lamp is generally used as a radiation source because relatively small and inexpensive systems having a relatively high light output can be made in that way. In fact, the compact gas discharge arc can collect a large quantity of light in a relatively small light beam.
A drawback of the known illumination system is that the lifetime of the gas discharge lamp is very much dependent on the power supplied to the lamp. For current UHP lamps, for example of the Philips trademark, the lifetime is approximately 4000 hours, for example for a power of 100 watts. When increasing the power, the lifetime will become undesirably short.